Gas turbine engine compressors

A fourth, unusual, type is the free-piston gas generator, which combines the functions of compressor and combustion chamber in one unit.

Through the compressor, the flow area decreases and the blades get smaller and smaller from stage to stage and this compensates for the increase of air pressure and density, creating a constant axial velocity.

The diffuser is a fixed or static component that escorts the air flow after leaving the impeller.

The impeller and the diffuser contributes about 65% and 35% of the total pressure produced in the centrifugal compressor respectively.

It is much more difficult to produce an efficient multistage centrifugal compressor because the flow has to be ducted back to the axis at each stage therefore most high-compression jet engines incorporate multi-staged axial compressors.

But if only a moderate amount of compression is required a centrifugal compressor is much simpler and efficient to use.

If the HP compressor pressure ratio exceeds about 4.5:1, then the unit will probably have variable geometry (i.e. variable stators) on the first few stages, to make the surge line on the compressor map more shallow, to accommodate the shallow working line.